It is known that single phase electrical supplies such as used in various residential, commercial and industrial applications can be designed to avoid shock hazards by departing from conventional practice and using a system which is isolated from ground. It is also known to provide circuitry for detecting leakage current between such an isolated system and ground. However, the systems which have been designed for single phase operation have not been suitable for operation in three-phase supply systems such as used in industrial applications. Moreover, the advantages of protection of three-phase systems and problems involved therewith have not been fully recognized and dealt with in the prior art.